Touch sensing, which involves sensing a contact of an object such as a human body and a stylus with a touch panel, is one of important technologies in a touch panel device.
One known method of touch sensing is a projected capacitive touch technology, which senses changes in static capacitances of sensing electrodes formed in a touch panel. The projected capacitive touch technology includes mutual-capacitance touch sensing and self-capacitance touch sensing.
The mutual-capacitance touch sensing is a touch sensing method which involves sensing mutual capacitances formed between drive electrodes and sensing electrodes provided in the touch panel. For example, Japanese Patent Application Publication No. 2014-106866 A discloses a mutual-capacitance touch sensing technology.
The self-capacitance touch sensing is, on the other hand, a touch sensing method which involves sensing self-capacitances of sensing electrodes. More specifically, self-capacitance touch sensing senses the self-capacitance of each sensing electrode, which is the sum of the capacitance formed between each sensing electrode and the circuit ground and the capacitance formed between each sensing electrode and a conductor (e.g. as a human body) located near each sensing electrode. Since an increase in the self-capacitance of a sensing electrode implies that a conductor is located near the sensing electrode, touch sensing can be achieved by sensing the self-capacitance of each sensing electrode.
Generally speaking, the self-capacitance touch sensing is not considered as being suitable for multi-touch sensing (that is, sensing of simultaneous touching at multiple positions of a touch panel), and therefore is not considered as being suitable for an application which requires multi-touch sensing, such as touch sensing of a touch panel mounted in a panel display device. According to a study by the inventor, however, multi-touch sensing can be achieved in a self-capacitance touch sensing technology by using an appropriately-configured touch panel and touch controller. According to the study by the inventor, the self-capacitance touch sensing technology is rather advantageous for reducing the volume of a user interface device including a display panel (such as a liquid crystal display panel) and a touch panel, because integration of a display panel and a touch panel is easier in the self-capacitance touch sensing technology.
One requirement of a touch panel device using self-capacitance touch sensing is improvement in the sensitivity of touch sensing. For example, a high sensitivity is desired especially when a touch panel device is operated with a stylus or a glove. Current technologies, however, do not achieve a sufficient sensitivity in self-capacitance touch sensing.
Another requirement of a touch panel device using self-capacitance touch sensing is power consumption reduction. Power consumption reduction can be especially significant when the touch panel device is installed in a mobile terminal which includes a battery as the power supply.
Japanese Patent Application Publication No. 2014-211850 A discloses a technique for improving a touch sensing performance with respect to a touch panel device which performs mutual capacitance touch sensing. It should be noted however that the technique for improving the touch sensing performance, which is used in the mutual capacitance touch sensing, cannot be applied to self-capacitance touch sensing, because the circuit configuration and the touch sensing principle are completely different between mutual capacitance touch sensing and self-capacitance touch sensing.